Contents:

Chapter 1. Introduction.
Chapter 2. The Sun's neighborhood, and larger.
Chapter 3. Types (classes) of stars.
Chapter 4. Methods to detect Exo planets.
Chapter 5. A note on the Habitable Zone.
Chapter 6. Types of Exo planets and the host star's "metallicity".
Chapter 7. A survey of the most facinating Exo planets, found sofar.

Chapter 1. Introduction.

Planets at other Stars... That means: other Solar systems...

In the past, most astronomers had little doubts, that lots and lots of other stars have accompanying planets too.

The first experimental data of a planet orbiting a starsystem, surfaced in 1988. The starsystem was "Gamma Cephei", which is
located at a distance of 13.8 parsec (pc), or 45 lighyears (ly) from our Sun.
But, proof was not fully conclusive at that time. For that specific planet, convincing data appeared much later, namely in 2002.

Starting from the early nineties of the former century, the "exo planet hunt" really got momentum. Ofcourse, in part this was also due to new sophisticated methods
and instrumentation. But, the suspected discovery of the planet at Gamma Cephei, was probably an eyeopener too.

The first few objects that were found, (a few years after the discovery of the planet of Gamma Cephei), were "planet-like" objects around pulsar-like objects.
These were great discoveries ofcourse, but we rather would have found planets around a real "Sun", that is, planets around another star.

Finally, in 1995, that happened at the confirmation of a "Jupiter" type of planet around the star "51 Pegasi" (at a distance of 15.6 pc or 51 ly).
That discovery is often viewed as the first alien planet that was found around another true (mainstream) Star. Remember, the validity
of the planet around "Gamma Cephei", was not confirmed before 2002, and it may be seen as the true very first discovered exoplanet.

Then, things went gradually faster and faster.

Up to 2006, almost all observations were 'Earth based'. But there were also early observations from the Hubble telescope, like in 2001,
where Hubble was used to analyze the atmosphere of HD 209458 b (which was found earlier in 1999).
In 2003, Hubble analyzed white dwarf stars in M4, which is a globular cluster of stars. One such white dwarf has been found to be a binary star
with a pulsar companion, and a planet orbiting it with a mass of about 2.5 times that of Jupiter.
This particular planet might be the oldest exoplanet yet found, since it is estimated to be about 12.7 billion years old.
Hubble used the "directly imaging" technique for it's discoveries.

Then, the French satellite "CoRoT", started to collect evidence for exoplanets, as of februari 2007.
The probe used the socalled "transit method", which will be explained later. Although the number of exoplanets found is somewhat on the low side (about 32),
it's mission is widely regarded as a huge success.
One of the great successes of CoRoT, is the discovery of socalled "super Earth" like planets (which mass is between 1-10 earth mass).

But 'Earth based' researchers were quite busy too! One major success, was the discovery of "Gliese 581 c", which was the first low-mass extrasolar planet found,
near the socalled "habitable zone". It was discovered in April 2007.
The "habitable zone" is that "band" around a star, where a planet does not meet too hot, or cold conditions, so that if all other conditions are fine too,
the planet (in principle) may sustain life.

Note:
The "habitable zone (or the Goldilocks zone) is officially defined like to be the region around a star where planets with sufficient atmospheric pressure,
can support liquid water at their surfaces.

In the years 2008/2009, the number of exoplanets found was already around 800 or so. In fact, that's an amazing number ofcourse.
We have to keep in mind that observing a planet around another star is a challenge indeed. At such large distances, a planet is almost
undetectable by it's own radiation (light), and is ofcourse completely blazed away by the light of the host star itself.
That's why "direct detection" can only be used in just a few cases. In chapter 4 we will see the methods used in detecting exoplanets.

In 2009, the "Kepler era" began. The Kepler space observatory, was launced in march, 2009. If you like a "productive machine", then Kepler it is.
Up to this date (januari 2015), it has found over 1000 confirmed new exoplanets, with a couple of thousends candidate objects!

Up to this point, the counter stands on over 1850 confirmed exoplanets, where quite a few are even located in the "habitable zone".

But Kepler uses a limited "target area", due to several reasons. Also, it might by now be running on it's last legs, since some
technical issue's came up (however, it's possible that some clever workarounds will be applied).

It's successor will be the "Transiting Exoplanet Survey Satellite" (TESS). This will be a space telescope (sattelite) for NASA's exoplanet search,
which is scheduled for launch in 2017.
It's target area will be very wide, and it will have lots of other exiting features.

I plan to create the following chapters: in chapter 2, we see some facts of our Sun's local neighborhood, which means which famous stars
are located near the Sun (say, within 250 ly). Then, we see how that fits in our own Milky way.
Next, I think it's of importance to take a look at the "mainstream" stars. What I mean here, is that we learn some facts about for example
F type stars, red dwarfs etc.. In chapter 4, we will take a quick look at the methods used in detecting exoplanets.
Finally, chapter 5 will take us to a survey of facinating exo planets where the accent surely lies on planets in the "habitable zone".

There are some nice appendices as well. Please take a look at them. You will find catalogues, starmaps, and some nice "simulations" too!

Chapter 2. The Sun's neighborhood, and larger.

Even Kepler only "sees" a modest area of our own Milky way. Figure 1 below, tries to illustrate this fact.

Figure 1 depicts our Milky way, the Spiral Galaxy which our own Solar system is part of. The Milky way is estimated to house about 100x10⁹ to 400x10⁹ stars,
and the visible "disk" spans about 100000 ly.
Ofcourse, the "internet Wiki (wikipedia)" can provide us with much interesting facts on our Milky way.

Most discovered exoplanets, are within 1000 ly distance from the Sun.

Fig 1. Illustrating the "target area" of the Kepler space observatory (illustration by Jon Lomberg).



Now, it's nice to appreciate the scale of things, even if that is not directly related to the "study" of facinating exo planets.
But, I believe any note on a particular subject, should have some attention for the context where this subject "lives" in, so let's spend a few
words on the Sun's neighborhood, the place that this neighborhood holds in the Milky way, and the place that the Milky way holds in
the stucture of "super clusters" in our universe.

2.1. Lightyears and parsecs.

lightyear:

A very common measure of distance in astronomy, is the lightyear (ly). But what is it?

The nearest star to the Sun, is "Proxima Centauri", which is about 4.2 ly away.
A "lightyear" (ly), is the distance which light travels in one year. This is an enormouse distance, when expressed in miles or km's.
In Vacuum, light travels with a speed close to 300000 km/s, so the distance Earth-Moon is covered in slightly over one second.
So in one year, light travels a distince of:

60 x 60 x 24 x 365 x 300000 = 9460800000000 km = (about) 9.4 x 10¹² km

To put that into perspective: suppose you have a very fast moving conventional space probe, moving with 20 km/s, then it will cover
a distance of one lightyear, in 15000 years !
In such a setting, a ride to "Proxima Centauri" will take more than 60000 years.

Parsec:

Ofcourse, we all know that the Earth revolves around the Sun. The distance from the Earth to the Sun, often called the "AU" (Astronomical Unit),
is "only" about 150 million km, which extremely short compared to a lightyear (1 AU is about 9 light minutes).
However, if an observer watch the stars at a certain moment, and compares that with an observation done 6 month later, then for nearby stars
it may appear as if they have "shifted" against the background stars (which are much further away).

In figure 2, you see a nearby star (say for example 20 ly away), and how it is observed from Earth, and 6 monts later.
Ofcourse, the illustration is highly exaggerated, since the distance Earth-Sun is much much smaller compared to the distance
from the Sun to the star ("d" in figure 2). Indeed, in reality the lines "L" and "d" are almost parallel.

Fig 2. Illustrating the "parsec".



The "parsec" is defined in the following way: it's the distance "d", if and only if, the angle between "L" and "d" is one "arcsecond" which is 1/3600 part of a degree.
The parsec turns out to be aproximately 3.26 ly.

The parsec is used by astonomers just like the other unit, the lightyear. The parsec has (or had) some additional value when trying
to determine the distances of (relatively) nearby stars. If you measure the angle to the star when the Earth is at opposite places (6 months later)
in it's orbit, you can say something about the distance of such stars. The method only works with (relatively) nearby stars, otherwise they are so remote
that they stay "fixed" in the background.

Since the parsec is used in some technical and popular documents, I think it's nice to know that it is a distance of about 3.26 ly.
So, the nearest star, Proxima Centauri, is about 1.3 parsec away (4.2 ly).

2.2. Neighborhood of our Sun.

In appendix 2.1, you will see some great maps of the Sun's local Neighborhood, meaning nearby stars, like those who are within 20ly,
or even those who are within 250ly.

Although it is not neccessary at all (since appendix 2.1 provide links to all of them), I put a couple of them here too.

You know, I absolutely love these sorts of maps, and that's probably why I have put them here too. My samples here are from
that great site "www.atlasoftheuniverse.com" (Richard Powell), which regards it's pictures as "free" as long as you make sure that you
mention where they came from.

As said before, these maps are fabulous. There is just one thing: they show the brighter stars, meaning that the smaller red dwarf stars
are often not shown on those maps, especially on maps showing a region larger than 50 ly. It can be explained why that is so.

First, the the smaller red dwarf stars, are very abundant, en thus such a map would be very "cluttered" when those stars were put
on the maps too. And remember, often a map wants to illustrate the bright and famous stars which are around in the Sun's local Neighborhood
(like Sirius etc..), which are all very bright stars indeed. The smaller red dwarf stars, are not exactly the most notable stars in the skies,
and therefor they are often left out. So, for example, Gliese 581, which is a very interesting (for us) red dwarf, will not be shown.
Thus the larger and very bright stars (like Sirius, or the giant Arcturus) are well-known to the public, and thus "cries" for a place on such maps.

Secondly, since the Sun is a G type star (in the socalled Spectral types of stars), before the second half of the 80's,
it was often suspected that the F and G type (yellow-ish) stars (like the Sun), would be the best places to host exoplanets.
That seemed rather obvious, since our own Sun (a G type spectral class), hosts quite a few planets.
As it turned out, actually, from late 90's up to this day, that the older red dwarf stars, are great places to hunt for exoplanets.
We will talk about that later on (why red dwarf stars are such great places).

For now, I would like us to take a look a two samples of "starmaps" (from "www.atlasoftheuniverse.com", R. Powell):

Fig 3. Illustrating "nearby" stars < 12.5 ly (from "www.atlasoftheuniverse.com").




Fig 4. Illustrating "nearby" stars < 250 ly (from "www.atlasoftheuniverse.com").



If you look at figure 4, that region, would be located within the small "circle" in the center of figure 1.

2.3. Large scale structure.

Up to now, most discovered exoplanets are within 1000 ly from the Sun. There are a few exeptions ofcourse,
like for example "MOA-2008-BLG-310-L", which is about 6000 pc distance away. That is really quite far.

All discovered exoplanets sofar, are located in our own Milky Way. To observe exoplanets in another Milky way, is practically, impossible.
Ofcourse, in science, it's not advisable to use the term "impossible", but it's likely that most astronomers would agree (for now) with the above.

Our Milky way, is member of a small cluster of galaxies, called the "Local Group", where most of them are "dwarf" systems (compared to our own Milky way).
Most notable is M31, which is the (large) Andromeda spiral galaxy. It's about 2.2 Mly (or 2.200.000 ly or 2.2 million ly) away.
M33, the "pinwheel" galaxy, is around 2.5 Mly away, and that's a much smaller spiral galaxy compared to Andromeda and our own milky way.
The local Group is about 5 Mly to 10 Mly across, and it is illustrated in figure 5.

Fig 5. Illustrating the "Local Group".



The Local Group is part of a "Super Cluster". As of september 2014, astronomers discovered a coherent system (in terms of velocities),
which is about 500 Mly accross, and contains about 100000 Galaxies. It's named the "Laniakea" Supercluster".

Before Laniakea, astronomers considered the Milky way to be part of the "Virgo Supercluster". This view is not abandoned really, and the Virgo Supercluster
is usually nowadays regarded to be a part of the "Laniakea Supercluster".

Let's take a look at a "Virgo Supercluster" first, which is illustrated in figure 6:

Fig 6. Illustrating the "Virgo Supercluster" (source: "wikipedia").



In figure 6, somewhat above the center, the "Local Group" is located. About 50 Mly away, is the "Virgo cluster", while the whole
"region" (of figure 6) is called the "Virgo Supercluster". Note all the other local clusters like for example the Fornax cluster and Eridanus.

As discussed before, most astronomers nowadays, like to view the "Virgo Supercluster" to be a fair part of the "Laniakea" Supercluster",
which is illustrated in figure 7.

Fig 7. Illustrating the ""Laniakea" Supercluster".



"Green" regions in figure 7 denote larger concentrations of galaxies. The region within the curved line,
actually is the "Laniakea" Supercluster" which is about 500 Mly across, and it contains an estimated 100000 galaxies.
Figure 7 is of a different "format" as the other figures. Here, the picture is rendered on the basis of velocities.

If we would zoom out much more, say, to a scale of about a few Giga ly, astronomers suspect, and it's almost proven, that the clusters
of galaxies are concentrated in "filaments" surrounding larger voids. Figure 8 tries to illustrate that fact.

Fig 8. Illustrating the ""filaments" (strings) of galaxy clusters, viewed from superlarge perspective.



The "large scale structure" of the Universe is not the subject of this note. However, I think it's important that we get a feel
of perspective. I you would look at figure 6 again, showing the Virgo Supercluster, would you doubt that there are literally countless exoplanets?
No way. Even in the Sun's neighborhood, we "just have" discovered a couple of thousends, and many more are out there, waiting to be found...

2.4. Back to our own Milky way.

Figure 1 gives a nice view on our Milky way. Astronomers differentiate between a few "types" of "milky ways" (or better: galaxies).
Although sub-classifications exists, on a high level, astronomers classify galaxies as to "elliptical-", "irregular-", or "spiral galaxies".

Not withstanding those classifications, other characteristics set some galaxies apart, like for example the Seyfert type of galaxies, which
are believed to have an enormous black hole in the center, and those galaxies expose high levels or radiation, when compared to a "normal"
spiral galaxy.

Our Milky way is a spiral galaxy. It has a more or less flatter disk with a number of spiral arms containing young and old stars,
together with many regions of gasses and dust.
The milkyway has a densely populated Center, and it has a spherical "halo" which has a number of smaller spherical
clusters of stars known as "globular clusters".

The center is believed to house a giant black hole too, of about 4 x 10⁶ solar mass.

Our Solar system is located in one of such spiral arms, of about 27000 ly distance from the center of the Milky way.
The pictures below try to illustrate this a bit.

Ofcourse, this humble note can hardly say anything of relevance of the Milky way.
Therefore, if you are really "sort of new" to this, you best do a websearch on our Milky way, and I'am sure that you will find
lots and lots of astonishing facts.

Fig 9. Illustrating our "Milky way" (source: en.wikipedia.org/wiki/Milky_Way#mediaviewer/File:Milky_Way_Arms.svg).



Next, we have two short chapters. In chapter 3, we will see some information on types of Stars and how they are devided into spectral classes.
This is important, since we need to appreciate some specs like age, abundance, mass, temperature, influences on the "habitable zone" etc..
Then, in chapter 4, we will meet the most common methods that enables us to find exoplanets.
Following that, in chapter 5, we will see some usefull facts about the "Habitable Zone".
Then, finally, in chapter 6, we will do a survey of the most facinating exoplanets (as is known at Jan/Feb 2015).

Chapter 3. Types (classes) of stars.

3.1. Star spectral classes.

I am sure you often have watched the skies at a clear night. There sure are many different stars out there.

For example, if know how to locate the "Big Dipper" constellation (Ursa Major), and imaginary prolong the "arc"
a few times, you will see "Arcturus", a orange Giant star, at a distance of about 37 ly.
Let's get some facts of this Giant: (you might try to locate Arcturus in fig. 3)

Star: "Arcturus"
Colour: Orange
Mass: 1.08 x mass of the Sun
Surface Temp: 4286 K
Radius: 25.4 x radius of the Sun (20 million km, while the Sun has a radius of 0.7 million km)
Spectral Type: K0III
Age: about 7.1 Gyr (7 Giga years, 7x10⁹years)
Distance: about 37 ly

Well, this star then has a similar mass as the Sun has, but it's quite big: if the Sun would be a small marble,
then Arcturus would be the size of a football. However, many larger stars exists. For example, the orange Giant, "Aldebaran",
at a distance of 65 ly from the Sun, has an estimated radius of 44 x R_Sun.

Both stars, are, so to speak, practically "around the corner", since distances of about 50 ly, are ofcourse very small compared
to the dimensions of the Milky way.

Note that those orange/red Giants have a relatively "low" surface temperature. Our Sun has a surface temperature of about 5,700 K,
but in it's core (where nuclear fusion takes place), the temperature is around 20 MK.

Large White/Blue coloured stars are around too. These giants often have a higer mass, like for example more than 3 times that of the Sun,
or, way way more. The heavy White/Blue stars, are relatively young, and they "use fuel" fast!

Here is a famous example: Rigel.

Star: "Rigel"
Colour: blue
Mass: 21 x mass of the Sun
Surface Temp: 12100 K
Radius: 79 x radius of the Sun
Luminosity: 120000 x the Sun
Spectral Type: B8
Distance: around 860 ly
Age: about 8 Million years

Do you notice the large differences between those hot blue Giant stars and the Orange/red Giants?
Thee blue ones are very massive, very young, shortlived, and consume their fuel very fast, and will live "short".
For example, Rigels has just a few millions of years to go.
Compare that, for example, to Antares, wich is already 7 billions of years around.

Superlatives in astronomy, are never far away. What to think of "Eta Carinae"? This really is a blue supergiant,
about 8,000 light-years away, but only a fraction dimmer in our skies when we compare it to Rigel.
Eta Carinae will probably detonate in a large Supernova explosion within 100000 years, or somewhat longer.
Good thing it's at a large distance. If it were, for example, located as close as Antares, we would have a serious
problem with such a Supernova explosion.

Astrophysicists have studied stars for a long time now, and much facts are known. Ofcourse, continuous studies go on, but it's
amazing how much is already proven to be facts. You can easily study astrophysics for years, which illustrates again what
a humble note this actually is.

One nice sort of diagram, is the "Hertzsprung–Russell" (HR) diagram. It relates the Spectral class (colour) of a star to it's surface temperature
and it's luminosity. Figure 10 shows an example:

Fig 10. "Hertzsprung–Russell (HR)" diagram.



Note how the colour (or spectral class) of stars is divided into the "O B A F G K M" spectral types, thus ranging from hot and blue (O, B)
to less hot and reddish (like M class).
Note how the HR diagram, show the socalled "mainstream" stars, which are the types in the "band" in the diagram.
Exceptions remain however, like the "subdwarfs", "white dwarfs", "brown dwarfs", and other types of stars.

Also note that our own Sun is a typical yellowish "G type" of star.

A nice way how starting astronomers remembered the different spectral classes, is by using the rhyme: "Oh Be A Fine Girl Kiss Me".

3.2. Binary (or multiple) star systems.

Our Sun is "on it's own". Indeed, the nearest star is about 4.2 ly away. Typically, it seems that in this sector of our Milky way,
stars are seperated by a few, or some tens, of lightyears.

Well, many "binary systems" were found too. These are two stars in close vincinity, and they move around their center of gravity.
Not only binary systems were found, but systems with 3 stars (and even more) in close vincinity are not uncommon too.

The two "partners" could be of any spectral class (as we have seen above), however, quite often one of them is a "white dwarf".
Sometimes a "pulsar" is found too. A "white dwarf" is a star at the very end of it's life. It could have been a normal F or G type once,
but now it has become a "stellar remnant".

White dwarfs are those stars whose mass is not high enough to become a neutron star (or even a black hole).
A white dwarf is small and quite dim. It's generally accepted in the scientific community, that the final stage (or almost final stage) of our Sun,
is to become a white dwarf too. Maybe a white dwarf will eventually become a "black dwarf".

This humble note will not go into stellar evolution, and astrophysical processes. My main theme here is "exo planets", and what I try
to do in chapters 2,3, and 4, is to provide for some "reasonable background" info, to appreciate/understand exo planets better.

A very heavy star (like the very hot and heavy O stars), may end it's life in a Supernova explosion, and a socalled neutron star,
or even a black hole, may remain.

A "brown dwarf" is a star that failed to become a "real" star, due to lack of mass, and thus the Hydrogen fusion process did not get started in it's core.
Maybe, the really large giant Gas planets, much larger than our own Jupiter, are in some respects, quite close to "failed stars" too.

Just like the white dwarf, a brown dwarf might be quite often be a member of a binary, or triple, star system.

Back to Binary (or multiple) star systems. They are very common. For example, the Sun's neighbor Proxima Centauri, is a member of
of a triple star system (together with Alpha Centauri A and B).
Another famous star, "Sirius", at a distance of only 8.6 ly away, consists of Sirius A (the bright one) and a small dim white dwarf, Sirius B.

Ok, now let's see how the scientific folks try to detect "exo planets". Those cunning people have quite a few tricks for doing that.

Chapter 4. Methods to detect Exo planets.

There are a number of ways to find Exo planets. Using one technique alone, might lead to a false hit.
This is so, since using one method exclusively, let us usually find one property only of the companion, like the mass, or diameter.
That's why it is often strongly preferred that a combination of methods is used.

4.1. Directly Imaging (directly observed) Exoplanets:

This is just really "looking" at an object, or actually, using photographing techniques, at any suitable wavelength, that the scientists see fit.

Even at a relatively "short" distance of, say, 50 ly or so, to visibly "detect" a planet in another Solar system, is extremely hard.
Evidently, the little radiation of the planet itself, is blistered "away" by the light of the star.
Indeed, planets are ofcourse much dimmer than the star that they orbit.
So, often scientists use some wavelength (like infrared), and try to subtract the light that's due to the star, if possible,
so that what remains is from the planet.

This method, called "Direct Imaging" has been succesfully used in a few cases. At the time of writing, about 18 Exo planets were
found by this technique.

Here, the scientists try:

• to observe the light (of any suitable frequency/colour), produced by the planet itself, e.g. infrared from young planets,
• or they try to observe any reflected light (reflected starlight by the planet).

It's often said that the method is effective when young planets are observed. Then, they might emit sufficiently infrared light. If the distance from that planet to the star
is large enough, then the light is not fully drowned by the light from the star.
But, that is not to say that the method also have worked for "reflected" light from the star.
Actually, when I study articles on planets that were actually found, they are often suspected to be of an substantial age (that is: old), which conflicts a bit
with the statement above.

The planets that have been found using this method, are quite large too. The lightest one sofar, (Formalhaut b) is about 2 x M_earth.
Al the others are substantial more massive (for example like 5 x Mass of Jupiter).

- A nice example might be "HR 8799 b". which is an extrasolar planet located approximately 129 ly away, orbiting the star "HR 8799".
It has a mass of about 4 - 7 M_Jupiter, which is rather large indeed.
It was discovered in 2008 by Marois (et al), using the Keck and Gemini observatories in Hawaii, by direct imaging.
Interestingly, it was found In 2009, by re-analyzing all archives, that the Hubble Space Telescope had in fact (!) directly imaged HR 8799 b,
ten years earlier, namely already in 1998.

- Another great example is "Beta Pictoris b". This exoplanet is about 63 ly away. It's orbiting the star "Beta Pictoris".
It was discovered in 2008 by Lagrange (et al.), using the NACO instrument on the Very Large Telescope at Cerro Paranal in Chile.
It's mass sits between 4 and 11 M_Jupiter, and it orbits around 9 AU from Beta Pictoris.

By the way, do you notice that if a star is called "Star Name", that found planets are then consequently named "Star Name b", "Star Name c" etc..

Anyway "Beta Pictoris b" is interesting by the used variant of direct imaging, namely by utilizing reference star differential imaging.
It means that if the planet is really close to the star, then for a certain frequency range, the scientists "substract" the star's radiation,
which then might reveal the planet.
Sophisticated software is needed for such a process. An attempt was already undertaken as early as 2003, but it was not before 2008 that the reduction method
was advanced enough, to (finally) show that the planet was really there.

4.2. Radial velocity: Shifts in starlight frequency due to radial velocity changes:

The priciple of the "Radial velocity" detection method, is not hard to grasp.
If a planet orbits a star, then actually both orbit the center of gravity. Ofcourse, the star is almost always much
more massive than the planet. So, at best, you might say that the star "wobbles" (somewhat), while the planet has a wide
elliptical orbit. This is illustrated in figure 11.

Fig 11. The star "wobbles" around the center of gravity.



Now, we often cannot directly "see" the wobbling of that star. But undoubtly, you know the dopplereffect, which means a frequency shift
if an object is travelling towards you, and then moves away again, which alters the frequency of the starlight too.
This frequency "shift" can be measured with a very high precision.
And if the mass of the star is known, we can deduce the mass of the companion too.

If the companion then, is not too massive, it's likely to be a planet. But you understand that the very large Jupiter-like exoplanets
(like having 20 x Mass_Jupiter or so), are not simply to distinguish from another type of object, like a small brown dwarf star.

Collary: The Astrometric method:

In a few cases, the "wobbling" of the host star can be observed directly, if the star is not too distant, and the companion planet
is very massive.

4.3. Transit method:

It's fair to say that (up to this moment) most exoplanets were found using the "Transit method". Also the Kepler telescope,
which has found already over 1000 confirmed exoplanets, uses this technique.

Here too, the priciple of the "Transit method", is not hard to grasp. It's illustrated in figure 12.

Fig 12. The "Transit" detection.



When a planet slides across it's star, and we are in the line of sight of this system, the intensity of the star's light will
drop slightly. However, often this can be measured to a high precision. The "slope" of the drop in the curve, and how much
it is at the minimum, will (in many cases) tells us much about the planet's diameter.

Also, it's even possible that information is obtained from the atmosphere around that planet (if it has one),
since the stellar spectrum is known, and if absorbtion of certain frequencies occurs while the planet "is in front"
of the star, it may tell us more (if we are lucky) about the composition of the planet's atmosphere.

4.4. Gravitational Microlensing:

This method, "Gravitational Microlensing", has not been used much (sofar), in discovering exoplanets.
It's illustrated in figure 13.

Fig 13. The "Gravitational Microlensing" technique.



It's a fact that ElectroMagnetic radiation, like light, can be "bend" in a gravitational field around a massive object.
So, in some cases, we can see the following effect. Suppose we have a remote object, like a distant star.
Much closer to us, we have a second star, that's in the line of sight, between us and the remote star.
The light from the remote star might be bend due to the gravitaion of the "close by" star.
Now, suppose this star has a planet, then a small distortion might be found in the way the light of the remote star is bend.

It's a clever technique, and relies on sophisticated hardware and software components, but it has proven to work.

We have touched upon four well-known techniques here. However, there are some more techniques.
But this note does not pretent to be complete on subjects as presented in this chapter.
Indeed..., my main theme in this note, is a tour around the most facinating Solar systems.

It's also important to realize that the methods mentioned in 4.2 & 4.3, work better with the smaller and reddish K and M class of stars.
First, they "wobble" significantly more when a planet accompanies it (compared to a very massive star), and since they are also more dim,
in general, an exoplanet "stands out" more.
Secondly, we are very interested in M and K stars because planets in the habitable zone should be more easily detected with the techniques
of sections 4.2 & 4.3.

Well, we are almost there... (meaning that tour).
Just two little things more though..., before we start the tour. Let's take a further look at the "Habitable Zone" first.
And let's see what sorts (or types) of Exo planets have been found thusfar.

Chapter 5. A note on the Habitable Zone.

Remember from Chapter 3, that there are quite a few types of stars. Take a look at the Hertzsprung–Russell diagram again, in figure 10.
For example, from the young, and massive, and superhot O and B blue stars, we have the yellowish, smaller, and rather "average" F and G type of stars,
which are not so hot, and live long.
Then, we have the smaller and cooler and reddish K and M stars. These are very old, and are going to be around for a very long time.

If you would take a look at one of the catalogues of Appendix one, then you will find listings of the exoplanets that were found sofar.

You might pay some special attention to the host star's Spectral Class.
If you would do so, you should see that almost exclusivly F,G,K, and M spectral class stars are mentioned in those cataloques.

There indeed is hardly any O or B giant in such listing. Why is that so?

First, most of the detection methods listed in chapter 4, work better for the F,G,K, M (L,T) Spectral class stars.
Secondly, the O and B (supermassive, and superhot) stars, are not so abundant as the F,G,K, and M stars.

Our Sun is a "typical" G class star. It is likely that the F,G,K, and M class stars, are the best places to find planets at all,
and, planets which are in the "Habitable Zone".

Fig 14. The "Habitable Zone" in relation to the distance to the star and it's spectral class.



The O,B and A stars are not likely to have planets in the Habitable Zone. First, they are shortlived, say, from a few million of years
to a couple of hundreds of million of years. It's not likely that "favourable" conditions can arise on such planet in such a short time.
Secondly, they output so much radiation that any planet would have a "hard time" in that environment.

The best places are probably around the F,G,K, and M class stars. Figure 14 tries to illustrate the location of the Habitable Zone
around such stars. The hotter F and G stars, have an Habitable zone which is comparable to the orbit of the Earth.
For the smaller and cooler M type stars, the Habitable zone lies somewhat more towards that star.
For a larger, hotter F type, or even A type, the Habitable zone lies more outwards from that star.

From the literature, it can be found that most astronomers, like the M dwarfs a lot, in order to find planets in the Habitable Zone.
Those stars are very stable, and already have been around for a long time, and last but not least, are very abundant.

Chapter 6. Types of Exo planets and the Star's "metallicity".

In this chapter, you will find a few short remarks on the types of Exo planets found thusfar.

The planets in our own Solar system, are, to a certain extend, illustrative for what scientists have found as types of Exoplanets.
Indeed, some small rocky "Mercury like" planets have been found. Also, some planets were found that resemble Venus or Earth.
Quite a few "Neptune likes" and gas dwarfs have been found, and oufcourse the "Jupiter like" gas giants seem to be very abundant.
Many of those Jupiter-likes are very large like e.g. 3 to 15 x Mass _Jupiter.
Ofcourse, some of them are quite distant from their host star, but really lots of them are actually "hot Jupiters", orbiting near the host star.

However, some types of exoplanets have nothing like it in our Solar system, like for example the "Super Earths", which are rocky planets
with a mass between 1 and 10 x Mass _Earth.

Is there any sort of correlation between the types of exoplanets found, and the type of Host star?

Well, scientists seem to find exoplanets around the F,G,K, and M stars, while the O, B, and A classes are very poorly represented in the catalogues.
But keep in mind, as already said in chapter 4, that the current detection methods works best for the cooler and smaller stars.
But that's hardly a correlation.

But there seems to exist an actually correlation alright, based on the currently discovered exoplanets and their Host star's data.
The presence of rocky planets, gas dwarfs, or gas giants around a star seems to depend on the host star’s "metallicity".
A higher level of "metallicity" means that relevant levels of elements heavier than hydrogen and helium, can be found in the star itself.

Several studies have been performed. Some suggest the following "divide lines":

- Metal-rich stars are much more likely to harbour gas giant planets.
- The correlation is lowered for the for Neptunian like planets.
- terrestrial-size planets are often around host stars with a wide range of metallicities, but still pretty close to that of the Sun.

Some studies even suggest a "finer" division, on the mass and radius of the terrestrial-sized planets, and the relation to the
metallicities of the host stars.

Ofcourse, in "hindsight", it only seems logical the the initial composition of the dust and gas clouds, that will eventually
form a Solar system, will also be an important factor for certain types of Exo planets.

Many studies on such subjects, are in progress.

More info:

Three regimes of extrasolar planet radius inferred from host star metallicities (arxiv.org).
An abundance of small exoplanets around stars with a wide range of metallicities (starplan.dk).

Chapter 7. A survey of the most facinating Exo planets found sofar.

All of the material above, helps me a lot. It enables me to talk freely, for example, about lightyears, M type stars, transit detection method etc, etc...,
just to name a few, without problems (that is, if you have indeed read all the stuff in chapters 1 up to 5).

Ok, let's start our survey... It think it will be Great !

Case 1: "Kepler 444"

At the time of writing (jan/feb 2015), this is as fresh as it can get. And, it's quite remarkable.

The Kepler observatory (in orbit around the Earth), just found a Solar system which is immensely old.
The system is at a distance of approximately 117 ly from the Sun.

This time, an ancient Solar system is found, which is almost only "slightly" younger than the Universe itself !
The star is "Kepler 444", which is about 11.2 billion years old (11.2 x 10⁹ years).
The thing is, that the planets found, 5 of them for now, are now the oldest known system of terrestrial-size planets.

It's really old. For comparison, our own Sun (and Solar system) is about 4.6 billion years old.

Also: keep in mind that the "Big Bang" is believed to have happened about 13.6 billion years ago.
So what we have here, is that planet forming (and the birth of Solar systems), already took place,
when the "Milky way" was very young indeed.
Thusfar, five "ancient" planets have been found, orbiting Kepler 444.

The original article describing the recently found system (submitted at 26 januari 2015), was published in "The Astrophysical Journal",
but it can also be found in the scientific library "arxiv.org".
It's this one: "An ancient extrasolar system with five sub-Earth-size planets", which can be found here.

The data:

Star:	Kepler 444.
Mass:	about 0.76 x Mass of the Sun.
Age:	about 11.2 G years.
Spectral Type:	Orange main sequence star of spectral type K0.
Planets:	5 sofar: Kepler-444 b,c,d,e,f.
Properties of the Planets:	They range in size, the smallest comparable to the size of Mercury, and the largest to Venus. All five planets orbit their sun-like star in less than ten days.
Distance of Kepler-444:	117 ly

The planets are not likely to have ever supported live, since their orbit is really close to the host star,
and are not in the Habitable Zone.

Fig 15. Artist impression of Kepler 444 and it's companion planets. By: Tiago Campante/Peter Devine.



Remarkable details:

The system has rocky terrestrial-size planets. This, while at the very early age of the Universe, the heavier elements are very scarce.
According to the theory sofar, is that the lightest elements (H, He) were very abundant, in the early Universe. Gradually, hot blue stars emerged, and
while they are shortlived, and went with a "bang" (nova's/supernova's), thus produced the heavier elements (like e.g metals).

At very much later times, like when our Solar system was formed (4.6 billion years ago), also the heavy elements existed,
and it is generally accepted that those elements are actually from the nova's and supernova's detonations long ago.

Remarkably, Kepler-444 is a metal poor star, while having terrestrial-like planets, instead of Gas giants.

The host star seems to be part of the "Arcturus stellar stream", which seems to originate from the inner disk of the Milky way,
and presently "moves" out of the disk.
There are several suggestions made. First, planet formation at the very inner disk of the Milky way, might have been different
from what we have previously assumed. Or, as another suggestion, the "Arcturus stellar stream" might even have an extra galactic origin,
like from some dwarf galaxy nearby, or some open cluster nearby etc..

Further study is needed. It is expected that the discovery of the Solar system of Kepler-444, will have a large impact on
planet forming theories.


More info is available in the following articles:

Original article on "arxiv.org"
nasa.gov
phys.org
wikipedia

Case 2: "Gliese 581"

The "Gliese 581" system, is absolutely spectacular. It's not only that this host star indeed has some
remarkable planets (of which at least one "probably" sits in the Habitable Zone), but it is also a showcase on how "volatile" observations
can be, on occasions.

For about the latter statement: The Gliese 581 Solar system started out with the discovery of the planets Gliese 581 b,c,d^*.
This was followed up by the discovery of the planets Gliese 581 e,f,g.
However..., Gliese 581 d was quickly "doubted" for it's existence, so, the usual naming of planets in the order of discovery, like b,c,d,e... etc...,
is a tiny bit in disorder at Gliese 581.
So, in order of discovery, we have b,c,e (where the false hit "d" was replaced with "e"), then followed by the new planets d,f,g.

It's just a fact that some scientists still have some doubts on the d,f,g planets, having good reasons for that doubt.
This results to the fact that, to be fully sure, we best stick to the planets who are not doubted: and these are b,c,e.

But, the last words on the total number of planets in the "Gliese 581" system, are not spoken yet.

Still, even if only Gliese 581 b,c,e exists, it remains a facinating system. Let's first review some data on Gliese 581:


The data:

Star:	Gliese 581
Mass:	about 0.33 x Mass of the Sun.
Age:	Somewhere between 7 and 11 Gy.
Spectral Type:	Red dwarf with spectral type M3V (M type).
Planets:	3 sofar: Gliese 581 b,c,e. Possibly more.
Distance of Gliese 581:	20 ly

So, we are pretty sure of the existence of the planets Gliese 581 b,c,e. The existence of the new "d" planet is questionable,
however, some astronomers still support the fact it's really there.

There is a certain "curtain" of "fog" around the planets d,f, and g. There is quite some evidence, that stellar activity
and "debri" in the system, make observations pretty difficult. But it's a great opportunity, to "calibrate" methods at the same time as well.

Please take a look at figure 16 below. Here you see Gliese 581 in comparison with the Sun. Gliese 581 is a small and rather cool star,
so the "Habital Zone" is quite close to this Host star. Figure 16 tries to depict that fact.
You see the three terrestrial-size planets b,c,e relative to where the "Habital Zone" of Glies 581 is likely to exist.

Fig 16. Habital zone of Gliese 581, and the positions of the planets.



With respect to fig 16: Please note that "d" is still doubted to exist, although most scientist believe it does.

Now, lets focus on the planet Gliese 581 c, which really exists (fully confirmed).

Gliese 581c

It probably has a mass in the order of 5.5 X Mass _Earth. Scientists classify it as a "super-Earth" and it is sort of "terrestial" (Earth-like).
The planet is in, or very close, to the Habitable Zone, which makes it "super special". Could it support liquid water?
Some unconfirmed suggestions have been forwarded. If it would have a reflectivity such as Venus, then the surface temp would be
around -3 degrees Celcius. If it would have Earth-like reflectivity, then it would be around 40 degrees Celcius.
However, up to the present day, no water could be detected, but that is primarily due to technical difficulties to do so.

So, no single conclusive remarks can be made, that would support the fact that "c" is a candidate to support life, unfortunately.

I am afraid we still have to wait a bit for further, more accurate, observations are done on this very remarkable Solar system.

Remark:

Many articles, from various dates, present some different data on the exoplanets around Gliese 581.
A large number of the more recent articles, maintain that the existence of Gliese 581 b,c,e,d is confirmed, while 2 others are still doubtfull.
Those 4 planets would then be situated as is shown in figure 16.

Case 3: "Kepler-438"

Appendices:

1. Exoplanets: Catalogues, methods to detect.

1.1 Nice Catalogues:

1. exoplanet.eu
2. exoplanets.org
3. planetarybiology.com
4. openexoplanetcatalogue.com
5. caltech.edu (nasa listing)
6. Wikipedia: list of potentially habitable exoplanets
7. Wikipedia: list of exoplanetary host stars
8. planetquest.jpl.nasa.gov

1.2 Methods to find exoplanets:

1. Wikipedia: Methods of detecting Exoplanets
2. smithsonianmag.com: How to find an Exoplanet

2. Maps.

2.1 Sun's Local Neighborhood:

These are all maps of stars which are very close to the Sun (typically, less than 250 ly).
"Static" maps will always work. However, some "interactive maps" require a recent version of Java (so they might not work on your system).
Typical of such maps, is that they display the brightest stars, but do not show the smaller red M type stars, which are very abundant.

1. atlasoftheuniverse.com - stars within 12.5 ly (static)
2. atlasoftheuniverse.com - stars within 50 ly (static)
3. atlasoftheuniverse.com - stars within 250 ly (static)
4. kisd.de - stars within 14 ly (interactive)
5. newsfrombree.co.uk - stars within 20 ly (static)
6. solstation.com - several maps (static)
7. Wikipedia - Solar Interstellar Neighborhood (static)

2.2 Our Milkyway:

1. skysurvey.org (interactive)
2. spitzer.caltech.edu (interactive)

3. Simulations.

1. from "chromeexperiments.com" (interactive, online)
2. from "nasa/jpl/caltech", get a nice (installable) 3D program to see a simulation of nearby Exoplanets.

4. Archives.

1. eso.org: Archive of Exoplanets from ESO, and other great stuff

5. Other noteworthy stuff:

1. The Kepler target region of the Milky way (relative Milky way).
2. Close-up of the Kepler target region of the Milky way (relative Local Arms).

Ofcourse, any note from me, is free for use anyway you see fit.